1. Field of the Invention (Technical Field)
The present invention relates to wastewater remediation.
2. Background Art
The following processes pertain to primarily physical aspects of wastewater treatment and natural and artificial wetlands management. Most wastewater treatment techniques start with a primary filtration or sedimentation process. Primary filtration processes generally take advantage of density mismatches between the fluid phase and the suspended phase. Gravity causes materials having densities greater than the fluid phase to settle downward whereas materials having densities less than the fluid phase will rise. Typically, the greater the density difference or buoyancy, the greater the particle acceleration through the fluid. However, other phenomena may affect a particle's acceleration through a fluid medium. For instance, attractive forces may cause particle agglomeration or flocculation. The agglomerate or flocculent may have a density different than individual particles. In addition, hydrodynamics effects will change as the size of the agglomerate or flocculent changes. Particles may also experience hindered settling. Chemical additives or mixing can be used to promote or prevent agglomerate and flocculent formation. Another phenomenon, hindered settling, occurs when the particle number density and corresponding excluded volume is high. Hindered settling is comparatively analogous to fluid flow through a porous medium. Although seemingly simple, primary filtration depends on many variables and an a priori calculation of settling rates is not straightforward.
Fluid can flow through a porous bed due to a pressure difference, a potential or kinetic energy difference, or surface tension related capillary effects. Usually, these driving forces are related and intertwined. Consider a fluid in a porous medium heated by a geothermal source. As the temperature of the water increases, the density will decrease. Although relatively incompressible, water does stratify due to buoyancy effects, hot water rises and cold water sinks. Further, for most fluids at subsonic velocities, a pressure gradient is established where flow occurs from a high pressure to a low pressure region.
Soil can be thought of as a porous medium. For water in natural and artificial soils, flow is driven by a gravitational potential gradient and a matric potential gradient. Vertical fluid flow through natural and artificial soil beds depends to a large extent on the hydraulic conductivity which in turn depends on the matric potential. The hydraulic conductivity is a nonlinear function of the degree of saturation. It is maximum for a saturated bed and roughly one-millionth of the maximum when the bed is one-fourth saturated. The saturated conductivity also depends on the soil texture. Coarse soils have a high saturated conductivity compared to fine soils. Flow in saturated soils relies almost exclusively on the gravity potential gradient as a driving force for flow. The matric potential gradient dominates only during infiltration, especially for horizontal soil beds. For predominantly horizontal soil flows, the gravity potential gradient tends to zero, leaving the matric potential gradient as the main driving force for water infiltration. Infiltration plays an important role in several wastewater processing steps. Knowledge of natural or artificial soils hydraulic conductivity allows for a prediction of the final or steady-state infiltration rate. When combined with an analysis of the rate of water storage in the soil, start-up and periodic operation of a wetted soil bed may be predicted. Such an analysis is helpful when introducing wastewater into a wetlands system and for estimating the rate of soil contamination if a breach occurs in a containment barrier.
Evaporation is another physical process that impacts flow in open soil beds or wetlands. Fluid evaporation from the soil bed surface may cause the concentration of constituents in the soil to increase. On a hot summer day, one square meter of vegetated ground surface may lose over 10 kilograms of water (about two and a half gallons). Evaporation from a saturated soil bed may be substantial and exceed that for a typical vegetated ground surface. If the rate of evaporation exceeds the inlet flow rate, then, given sufficient time, the soil bed will dry. When plants are present, rates for transpiration and plant water uptake need to be estimated and added to the water loss rate from evaporation. The process of transpiration and plant water uptakes starts with the roots. Once in the plant, water moves through the xylem and into the leaves. Water evaporates from the leaves through the sub-stomatal cavities, perhaps the main mechanism for the regulation of water in plants. However, the rate of water loss depends on the evaporative demand of the surrounding air. If the air is saturated, water loss is minimal.
Another physical factor affecting the operation of natural or artificial wetlands is wind. Wind accelerates evaporative water loss thereby cooling the system, introducing seeds of unwanted plant species, driving fluid flow through surface effects, and even destroying plant life. The structure of the wetlands and orientation to the wind may have a profound effect on wind velocity over the vegetated surface. However, as with most crop canopies, the wind velocity drops quickly from the canopy to the ground surface.
Taken together, physical phenomena have a great impact on the functioning of a natural or artificial wetlands system. Proper placement of an artificial wetlands is imperative to ensure an environment conducive to proper operation. Improper placement with respect to the physical surroundings may result in the formation of an adverse microclimate and microhabitat in which the wetlands will not thrive.
The crux of the remediation process relies on the ability of the flora to reduce chemical constituents or waste to nonhazardous forms or acceptable concentration levels. For photosynthetic flora, the process starts with solar radiation. Fortunately, indigenous plants have adapted their own mechanisms to regulate radiative energy uptake. However, these mechanisms can be negated when multiple plant species compete for the same available light. One solution is to choose a variety of plants having different radiation requirements, i.e., some species may thrive on radiation reflected from the surrounding rather than on direct beam radiation. For subsurface species that live in the aqueous phase, water turbidity plays an important role in attenuating radiation. Thus, regulation of the inlet turbidity may be required to ensure that subsurface species survive. In climates sufficiently removed from the equatorial regions of the globe, seasonal variations in radiation also need to be accounted for to ensure a proper degree of remediation. Lastly, radiation has the potential to heat the wetlands directly or indirectly. The degree of energy input into the system by radiation may be controlled through a variety of means. For instance, if the upstream settling tanks used for storage are painted black, the inlet stream will have a higher temperature. Overall, radiation is linked to many different variables that affect the performance of a natural or artificial wetlands system, most intimately through the regulation of photosynthetic flora.
The biological processes that degrade the chemical constituents in the wastewater stream rely on both plant and microbial species. Plants help to assimilate and store contaminants while providing a substrate for microbial growth. For leafy plant species, three factors must be favorable for a leaf to remain alive. Positive average net photosynthesis and non-lethal bounds on leaf water potential and temperature. Plant roots also require a sufficient supply of oxygen to survive. Aquatic plants have the ability to translocate oxygen from the upper leaf areas into the roots producing an aerobic zone around the roots. Oxygen can be supplied to plant roots by several mechanisms. Oxygen may be absorbed at the leaves and travel down the xylem to the roots, or oxygen may be directly introduced through the surrounding soil and water. In active transport processes, the plant expends energy while diffusive transport occurs under thermodynamically favorable conditions. For instance, diffusive transport is favorable when the oxygen concentration in the surrounding soil is greater than the concentration in the plant root. Alternatively, oxygen may enter the root through osmotic movements of water. Grass species typically require a dissolved oxygen level between 1 and 7 ppm.
Commonly used plants for constructed wetlands include common reed, cattail and hardstem bulrush. However, the common reed and hardstem bulrush are invasive and generally require annual harvesting, a labor intensive process. To create an economically favorable constructed wetlands operation, low maintenance and aesthetic plants are preferred. In addition, constructed wetlands may create microclimates and microhabitats that allow non-native species to grow.
Many organisms inhabit wetland environments including Group 1: bacteria such as zoogloca, protoza such as ciliatea, flagellate, rhizopoda and the like and larger organisms, ones that use sludge as food, including nematoda, rotatoria, oligochaeta, anthropoda, and the like. The larger organisms tend to be more sensitive to dissolved oxygen fluctuations.
The organisms play an essential role in the overall processing of the wastewater stream. When nitrogenous waste is present, typically in the form of animal and plant proteins and urea, the wetlands must facilitate the nitrogen cycle. To convert nitrogenous waste to nitrogen gas and fixed nitrogen in the form of nitrate salts, a combination of denitrifying and nitrifying organisms are needed. Nitrification involves conversion of nitrogen wastes such as urea, uric acid and ammonia to an oxygenated nitrogen species such as nitrites and nitrates. This step requires oxygen and relies on aerobic metabolism. Soluble phosphorous may also be reduced during the nitrification process as phosphorous is an integral part of the cellular energy generation cycle. Species such as nitrobacters and nitrosomonas facilitate the nitrification step. Such bacteria are typically known as lithotrophs and they derive their carbon from carbon dioxide. Nitrosomonas transform ammonia into nitrite while nitrobacter transform nitrite into nitrate. One potential problem arises in converting ammonia to nitrite; hydrogen ions are produced. A reduction in pH may prove unfavorable in some circumstances. To avoid a drop in pH, the alkalinity should be at least eight times the concentration of ammonia and ideally over 100 ppm. A pH around 8 is near optimal. This problem is more acute in fresh water than in sea water.
Anaerobic bacteria facilitate denitrification; the conversion of nitrites and nitrates to dinitrogen, an inert gas under most all environmental conditions. Most of these are known as facultative anaerobes since they can use nitrogen oxides instead of dioxygen as an electron acceptor in the terminal step of the electron transport chain. However, a carbon source is required for these bacteria to perform the denitrification step. Without a denitrifying step, nitrogen levels may increase to levels that are detrimental to the overall balance of the ecosystem. One particular condition that should be monitored is the anaerobic degradation of sulfur oxides. Anaerobic decomposition of sulfur oxides generates hydrogen sulphide, a foul smelling and corrosive gas.
For carbonaceous wastes, a certain level of nitrogen is required to accomplish conversion to biomass, carbon dioxide and water. Bacteria known as heterotrophs are responsible for the degradation of the major carbonaceous wastes in sewage. Most heterotrophic water purification processes reduce organic carbon to carbon dioxide via respiration. In most systems, nitrification will not proceed in the abundance of a soluble organic carbon source. As a general rule, the soluble carbonaceous BOD should be below 20 ppm (equal to 20 mg/l) for any significant degree of nitrification to occur.
Most microorganisms grow to a certain cell density at which time growth stabilizes, usually closely matching the rate of microbial death. Since wetlands function on the basis of microbial action, the wetland will also experience a lag phase before reaching a substantially steady operating state. During the start-up period, several processes occur including vegetative fill-in, root and rhizome development and growth, and microbial community establishment. In some constructed wetlands, the rocks or gravel serve as a substrate onto which microorganisms attach. When the microbial film thickens, microorganisms near the surface of the substrate may not get an adequate oxygen supply. Oxygen supply may be limited by the metabolism of the more exterior organisms or by the rate of oxygen transfer through the film. Some operations use forced water streams periodically to slough off the film and allow a new film to grow.
The following patents disclose information in related fields U.S. Pat. No. 5,486,291, entitled "Ecological Fluidized Bed Method for the Treatment of Polluted Water," to Todd and Shaw, discloses a fluidized bed remediation system for rapid nitrification and denitrification that introduces high pressure air to circulate buoyant particles. Compressors driven by wind or solar energy are used to provide the pressurized air. U.S. Pat. No. 5,527,453, entitled "Apparatus for Treating Dirty Water Aerated by Solar Powered Compressor," to Hachima, discloses a waste treatment device that uses a solar energy driven air compressor and Nitrosomonas and Nitrobacter. U.S. Pat. No. 5,156,741, entitled "Wastewater Treatment System and Method," to Morrison et al., discloses a remediation system that uses a submerged air pump to provide a dissolved oxygen concentration of at least 1 ppm to the roots of turf grass species. Typical operation involves recirculating over 50% of inlet wastewater. U.S. Pat. No. 5,087,353, entitled "Solar Aquatic Apparatus for Treating Waste," to Todd and Silverstein, discloses a series of at least three light-transmitting tanks containing photosynthetic bacteria, nonaquatic plants, and fish. U.S. Pat. No. 5,078,882, entitled "Bioconversion Reactor and System," to Northrop, discloses a three stage remediation system with a georeactor stage having optional air bubblers in a gravel filled bed 1 to 10 meters in depth. Specific organisms contained within the system may indicate the degree of remediation. U.S. Pat. No. 4,959,084, entitled "Combined Air and Water Pollution Control System," to Wolverton, discloses a remediation system for the treatment of wastewater and polluted air. The system uses a water flow driven aspirator to create an air flow into the system. U.S. Pat. No. 4,906,359, entitled "Solar Activated Water Aeration Station," to Berthold, discloses a solar driven aeration station for introducing oxygen into a body of water at a predetermined depth. U.S. Pat. No. 4,443,337, entitled "Biological Treatment of Waste Water," to Otani et al., discloses a remediation system that uses an air compressor to provide a supply of dissolved oxygen.
Prior researchers have focused primarily on specific aspects of microbial, plant and animal species without a keen focus on creating microclimates and microhabitats or utilizing the potential of a given environment. Environmental changes associated with the seasons often complicate operation as well.
The present invention utilizes an optimized remediation system. This system utilizes low rates of air/oxygen flow and aeration; the aeration rate facilitates simultaneous nitrification and denitrification; and the aeration can be operated at lower temperatures and prevent freezing.